Luton Airport, north of London, has been closed until at least 3pm local time on Wednesday after fire engulfed a terminal multi-storey car park.
The blaze began at around 9pm on Tuesday evening and caused “significant structural collapse”, said Bedfordshire Fire and Rescue Service.
Four firefighters and a member of airport staff were taken to hospital suffering from smoke inhalation and another firefighter was treated at the scene.
The car park, a short walk from the terminal building, had capacity for 1,200 cars. Hundreds were reported destroyed in the inferno.
Flights were immediately cancelled. Around 25,000 passengers are thought to have been affected.
On Wednesday morning, Luton Airport said emergency services remain at the scene and flights would be suspended until 3pm, having initially been paused until noon.
The Civil Aviation Authority said passengers should contact their airlines for information regarding their flights.
Footage posted on social media showed flames and smoke tearing through cars parked at the newly built airport car park.
Russell Taylor, 41, an account director from Kinross in Scotland, saw the flames after flying in to Luton from Edinburgh.
He told the PA news agency: “There were a couple of fire engines with a car ablaze on the upper floor of the car park at just after 9pm.
“A few minutes later most of the upper floor was alight, car alarms were going off with loud explosions from cars going up in flames.
“The speed in which the fire took hold was incredible.
In a statement at 3am on Wednesday, the airport said: “Emergency services continue to respond to a significant fire in Terminal Car Park 2, which has resulted in a partial structural collapse … Passengers are advised not to travel to the airport at this time.”
More than 15 crews tackled the blaze, with nearby residents advised to close their windows to avoid the smoke.
A brigade spokesman said: “One half of the structure is fully involved in fire and the building has suffered a significant structural collapse.
“Fire crews are working to extinguish the fire and prevent it from spreading to adjacent buildings and vehicles on air side of the airport.”
Describing the car park, Luton Airport’s website said: “Our brand-new multi-storey car park offering secure, on-site parking using just a five-minute walk to the terminal entrance.”